{"gene":"IGFBP4","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1995,"finding":"IGFBP-4 inhibits IGF action by preventing binding of IGF-I and IGF-II to the type I IGF receptor; coincubation with IGFBP-4 decreased binding of both 125I-IGF-I and 125I-IGF-II to bone cells and to purified type I IGF receptor in a dose-dependent manner, and IGFBP-4 had no effect on proliferation induced by IGF analogs with >100-fold reduced affinity for IGFBP-4.","method":"Radioligand binding assays, cell proliferation assays with IGF analogs of reduced IGFBP affinity","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro receptor binding assay with affinity-reduced IGF analogs as controls, multiple orthogonal methods","pmids":["7544787"],"is_preprint":false},{"year":1995,"finding":"The protease that cleaves IGFBP-4 cuts at the peptide bond between residues 120-121 (Lys-120/Met-121) within the non-homologous mid-region of IGFBP-4; site-directed mutagenesis of K120A rendered IGFBP-4 relatively resistant to cleavage, and the resulting 16-kDa N-terminal fragment lost the ability to inhibit IGF-1-stimulated thymidine uptake.","method":"Electrospray mass spectrometry, amino acid sequencing, site-directed mutagenesis (K120A), thymidine incorporation assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with purified protease, mutagenesis, and functional validation in one study","pmids":["7538115"],"is_preprint":false},{"year":1999,"finding":"The human osteoblast IGF-II-dependent IGFBP-4 protease cleaves at Met135-Lys136 as the sole cleavage site; deletion of residues 94-119 (which contain no cleavage site) blocked proteolysis, suggesting these residues maintain the IGFBP-4 conformation required to expose the cleavage site; protease-resistant analogs were more potent inhibitors of IGF-II-induced proliferation in osteoblasts (which produce the protease) than wild-type IGFBP-4.","method":"N-terminal amino acid sequencing, mass spectrometry of proteolytic fragments, deletion mutagenesis, cell proliferation assay","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 1 — multiple mutagenesis constructs, mass spectrometry, and functional cell assay","pmids":["10620067"],"is_preprint":false},{"year":2000,"finding":"The interaction between IGF-II and IGFBP-4 (not between IGF-II and PAPP-A) is essential for efficient IGFBP-4 proteolysis by PAPP-A; pre-incubation of IGFBP-4 with IGF-II followed by removal of free IGF-II allowed proteolysis, while pre-incubation of PAPP-A with IGF-II did not; IGFBP-4 mutants lacking IGF-binding activity but retaining the cleavage site were resistant to proteolysis regardless of IGF-II presence.","method":"Cell-free protease assay, PAPP-A antibody immunoprecipitation, IGF-binding domain deletion mutants, pre-incubation experiments","journal":"Archives of biochemistry and biophysics","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with pre-incubation experiments and PAPP-A-specific antibody depletion","pmids":["10898936"],"is_preprint":false},{"year":2001,"finding":"PAPP-A is identified as the protease responsible for IGF-II-dependent IGFBP-4 cleavage in human pregnancy serum; cleavage of IGFBP-4 by PAPP-A can occur without IGF in the absence of IGF but at a very slow rate; IGF-II enhances proteolysis by binding to IGFBP-4 (making IGFBP-4 the substrate), not by directly interacting with PAPP-A.","method":"Specific PAPP-A antibody immunoprecipitation/immunodepletion of protease activity, cell-free proteolysis assay, IGF binding studies with purified recombinant proteins","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with highly purified recombinant proteins and antibody depletion confirming identity of protease","pmids":["11522292"],"is_preprint":false},{"year":2001,"finding":"PAPP-A accounts for the predominant IGFBP-4 proteolytic activity in human pregnancy serum; immunoprecipitation with PAPP-A antibody completely abolished both IGF-II-dependent and IGF-II-independent IGFBP-4 proteolytic activity; serum PAPP-A enhanced IGF bioactivity in vitro by degrading IGFBP-4, an effect blocked by PAPP-A neutralizing antibody.","method":"Immunoprecipitation/immunodepletion of PAPP-A from pregnancy serum, cell proliferation assay with wild-type vs. protease-resistant IGFBP-4, PAPP-A neutralizing antibody","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 — antibody depletion with functional readout, replicated across multiple methods","pmids":["11158056"],"is_preprint":false},{"year":2002,"finding":"PAPP-A is the IGFBP-4 protease in human trophoblasts and decidualized endometrial stromal cells; IGFBP-4 protease activity was confirmed by specific immunoinhibition and immunodepletion with PAPP-A antibodies; proteolysis was IGF-II-dependent, and its physiological inhibitor proMBP was identified in trophoblast conditioned medium.","method":"Immunoinhibition, immunodepletion with specific PAPP-A antibodies, conditioned medium protease assay, ELISA for PAPP-A and proMBP","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 — orthogonal antibody methods (inhibition + depletion) identifying enzyme and inhibitor","pmids":["11994388"],"is_preprint":false},{"year":2008,"finding":"IGFBP-4 functions as an inhibitor of canonical Wnt signaling independent of its IGF-binding activity; IGFBP-4 physically interacts with Wnt co-receptor LRP6 and Frizzled-8 (Frz8), inhibiting Wnt3A binding to these receptors; this cardiogenic effect was attenuated by IGFs through IGFBP-4 sequestration.","method":"Co-immunoprecipitation of IGFBP-4 with LRP6 and Frz8, in vitro cardiomyocyte differentiation, Igfbp4 knockdown in vitro and in vivo, IGF-binding-deficient IGFBP-4 mutant, Wnt3A binding competition assay","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, loss-of-function, mutant protein, and in vivo validation in a high-impact journal","pmids":["18528331"],"is_preprint":false},{"year":1995,"finding":"IGFBP-3 functions as an IGF-reversible inhibitor of IGFBP-4 proteolysis; recombinant IGFBP-3 added to osteoblast conditioned medium inhibited 125I-IGFBP-4 proteolysis by 90%, and this inhibition was reversed by IGF-I or IGF-II; synthetic peptides spanning the highly basic, putative heparin-binding C-terminal region of IGFBP-3 mediated this inhibition, and heparin reversed the inhibitory effect.","method":"Cell-free protease assay with 125I-IGFBP-4, synthetic peptide competition, heparin reversal experiment","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted cell-free assay with synthetic peptides and defined inhibitor mechanism","pmids":["7499205"],"is_preprint":false},{"year":1997,"finding":"IGFBP-3, -5, and -6 all inhibit IGFBP-4 proteolysis through homologous highly basic, heparin-binding domains within the conserved thyroglobulin type-1 motif at their C-termini; synthetic peptides from these regions inhibited 125I-IGFBP-4 proteolysis by MC3T3-E1 osteoblast conditioned medium, and heparin reversed their inhibitory effects.","method":"Cell-free protease assay with 125I-IGFBP-4, synthetic peptide competition assay, heparin binding studies, IC50 determination","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 1 — reconstituted cell-free assay with purified synthetic peptides, mechanistically defined","pmids":["9165012"],"is_preprint":false},{"year":2001,"finding":"Systemic administration of wild-type IGFBP-4 increased bone formation parameters in mice by increasing IGF bioavailability through proteolysis of IGFBP-4; protease-resistant IGFBP-4 administered systemically did not increase bone formation or serum free IGF-I, whereas wild-type IGFBP-4 did, confirming that the stimulatory effect requires IGFBP-4 protease-mediated release of IGF.","method":"In vivo systemic administration of wild-type vs. protease-resistant IGFBP-4, biochemical bone formation markers (serum osteocalcin, ALP), serum free IGF-I measurement, local administration assay","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — in vivo loss-of-function with protease-resistant mutant and multiple bone formation readouts","pmids":["11356715"],"is_preprint":false},{"year":1998,"finding":"Overexpression of IGFBP-4 in smooth muscle cells (SMC) of transgenic mice caused smooth muscle hypoplasia in multiple SMC-rich organs, a reciprocal phenotype to IGF-I-overexpressing transgenic mice; IGFBP-4 overexpression was confirmed to be the functional antagonist of IGF-I in SMC in vivo.","method":"Transgenic mouse generation with SMC-specific promoter (SMP8-BP-4), tissue weight measurements, in situ hybridization, Western ligand blotting, double transgenic cross with SMP8-IGF-I mice","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis in double transgenic model with phenotypic quantification","pmids":["9564877"],"is_preprint":false},{"year":2002,"finding":"Protease-resistant IGFBP-4 (with substitutions 119-AAMAAVADASAMA-133 in the cleavage domain) is stabilized in vivo and causes greater growth inhibition of SMC-rich tissues than equivalent levels of native IGFBP-4, demonstrating that IGFBP-4 proteolysis is required for local IGF-I action in smooth muscle.","method":"Site-directed mutagenesis of IGFBP-4 cleavage domain, transgenic mice with SMC-specific expression, tissue weight measurements, Western blot quantification of IGFBP-4/transgene mRNA ratio","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — protease-resistant mutant validated in vivo, direct comparison with native IGFBP-4 transgenic line","pmids":["11923290"],"is_preprint":false},{"year":2004,"finding":"PAPP-A is the IGFBP-4 protease in MC3T3-E1 osteoblasts; immunodepletion of PAPP-A from MC3T3-E1 conditioned medium abolished IGFBP-4 degradation; IGFBP-4 degradation was enhanced by IGF-II and inhibited by mutation of basic residues at/near the PAPP-A cleavage site.","method":"Immunodepletion of PAPP-A from conditioned medium, IGFBP-4 degradation assay, site-directed mutagenesis of IGFBP-4 cleavage site, RT-PCR for PAPP-A mRNA","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 — immunodepletion plus mutagenesis confirming PAPP-A as the osteoblast IGFBP-4 protease","pmids":["15541345"],"is_preprint":false},{"year":2003,"finding":"Paracrine overexpression of IGFBP-4 in osteoblasts via an osteocalcin promoter transgene caused 25-fold increased calvaria IGFBP-4, reduced bone turnover (osteoblast number and bone formation rate ~50% of controls), global growth retardation, and disproportionate reduction of skeletal weight, attributed to IGF-1 sequestration.","method":"Transgenic mice (OC-BP4), Western ligand blot, quantitative bone histomorphometry, organ allometry","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 2 — tissue-targeted transgenic model with quantitative histomorphometry and phenotypic characterization","pmids":["12733722"],"is_preprint":false},{"year":2013,"finding":"SOX9 directly binds the IGFBP-4 promoter and transcriptionally activates IGFBP-4 expression in intestinal epithelial cells; IGFBP-4 was significantly downregulated in Sox9-deficient crypts and adenoma cells; SOX9-induced inhibition of cell proliferation was restored by a neutralizing IGFBP-4 antibody.","method":"Chromatin immunoprecipitation (ChIP) of SOX9 at IGFBP-4 promoter, reporter assay, Sox9 knockout mouse model, neutralizing antibody rescue experiment, immunolocalization","journal":"American journal of physiology. Gastrointestinal and liver physiology","confidence":"High","confidence_rationale":"Tier 2 — ChIP plus reporter assay plus loss-of-function rescue defines direct transcriptional regulation","pmids":["23660500"],"is_preprint":false},{"year":2013,"finding":"The anti-angiogenic and anti-tumorigenic activities of IGFBP-4 are located in its C-terminal fragment (CIBP-4) containing a thyroglobulin type-1 (Tg1) domain; CIBP-4 internalized into endothelial cells and glioblastoma cells, co-localizing with lysosomal structures, inhibited cathepsin B (CatB) activity, blocked EC tubulogenesis, and reduced glioblastoma tumor growth by 60% in vivo.","method":"C-terminal IGFBP-4 fragment production, in vitro cathepsin B activity assay, Matrigel EC tubulogenesis assay, in vivo biodistribution with Cy5.5 labeling, xenograft tumor model","journal":"Neoplasia (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 2 — in vitro enzyme assay plus in vivo tumor model with mechanistic domain mapping","pmids":["23633927"],"is_preprint":false},{"year":2016,"finding":"IGFBP-4 inhibits β-catenin signaling in the ischemic heart by targeting β-catenin (not LRP5/6), whereas Dkk1 exerts injury mainly by inducing LRP5/6 endocytosis and degradation; direct injection of recombinant IGFBP-4 into infarcted hearts protected against ischemic injury through β-catenin inhibition.","method":"Conditional cardiomyocyte-specific LRP5/6 and β-catenin knockout mice, surgical myocardial infarction model, recombinant protein injection, Tcf reporter assay","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with conditional knockouts plus recombinant protein intervention in vivo","pmids":["27803037"],"is_preprint":false},{"year":2017,"finding":"IGFBP-4 is required for adipogenesis; Igfbp4-null mice had decreased inguinal and gonadal white adipose tissue weight, reduced Pparγ expression, and impaired adipogenic differentiation of primary bone marrow stromal cells (BMSCs) and ear mesenchymal stem cells (eMSCs); loss of Igfbp4 blunted phosphorylated Akt increases during adipogenesis.","method":"Igfbp4 knockout mice, primary BMSC and eMSC culture adipogenesis assay, high-fat diet challenge, ovariectomy, Western blot for p-Akt","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 — genetic knockout with defined cellular and signaling phenotype across multiple assays","pmids":["28938423"],"is_preprint":false},{"year":2017,"finding":"Igfbp4-null mice exhibit marked sex-specific skeletal phenotypes; female knockouts had reduced aBMD/aBMC, lower trabecular BV/TV, higher osteoclastogenesis, and lower bone formation, while males had increased trabeculae with higher connectivity density and reduced mineralized surface; loss of Igfbp4 modulates mesenchymal stromal cell differentiation and osteoclastogenesis in a gender-specific manner.","method":"Igfbp4 knockout mice, micro-CT, histomorphometry, osteoblast/osteoclast culture assays, sclerostin ELISA","journal":"The Journal of endocrinology","confidence":"High","confidence_rationale":"Tier 2 — comprehensive knockout analysis with histomorphometry and cell culture mechanistic follow-up","pmids":["28184001"],"is_preprint":false},{"year":1992,"finding":"IGFBP-4 purified from bovine pulmonary artery endothelial cells crossed the capillary boundary of the isolated perfused rat heart and distributed preferentially in subendothelial connective tissue (connective tissue/muscle ratio ~20-27:1), unlike IGFBP-1, -2, -3 and IGF-I which preferentially localized in cardiac muscle.","method":"Perfused isolated beating rat heart, purified glycosylated and nonglycosylated IGFBP-4, tissue distribution quantification","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment in perfused organ model, single study","pmids":["1377125"],"is_preprint":false},{"year":2001,"finding":"The C-terminal basic region of IGFBP-4 determines its connective tissue distribution in the rat heart; chimeric IGFBP-4(3) (IGFBP-4 carrying the IGFBP-3 C-terminal basic region) redistributed to cardiac muscle like IGFBP-3, and IGFBP-3(4) localized in connective tissue like IGFBP-4.","method":"Chimeric IGFBP-3/4 protein construction, perfused rat heart distribution assay, microvascular endothelial cell binding assay","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 — domain-swap chimeras with functional tissue distribution assay, single study","pmids":["11517150"],"is_preprint":false},{"year":1994,"finding":"IGFBP-4 in the IGFBP-4 protease system of human osteoblast-like cells is regulated by TGF-β, which augmented IGF-II-dependent IGFBP-4 proteolysis and decreased IGFBP-4 mRNA expression; other factors (PTH, GH, insulin, calcitonin, glucocorticoids, sex steroids, 1,25-dihydroxyvitamin D3, EGF) had no significant effect on IGFBP-4 protease activity.","method":"Cell-free IGFBP-4 protease activity assay, Western ligand blotting with densitometry, Northern analysis for mRNA","journal":"The Journal of clinical endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 — cell-free protease assay with systematic hormone testing, moderate evidence from single lab","pmids":["7527411"],"is_preprint":false},{"year":2000,"finding":"Circulating N-terminal (Asp1-Phe122) and C-terminal (Lys136-Glu237) fragments of IGFBP-4 generated by in vivo proteolysis were isolated from human hemofiltrate; the N-terminal fragment retained significant IGF binding (Kd IGF-II=17 nM, IGF-I=5 nM), while the C-terminal fragment had very low IGF affinity (Kd IGF-II=690 nM, IGF-I>60 nM); disulfide bonding pattern of C-terminal fragment: 153-183, 194-205, 207-228.","method":"Affinity purification from human hemofiltrate, mass spectrometry, Edman sequencing, plasmon resonance spectroscopy, ligand blot, saturation and displacement binding studies, proteolytic digestion for disulfide mapping","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — biochemical characterization with multiple orthogonal methods, structural data from natural human samples","pmids":["10819974"],"is_preprint":false},{"year":1998,"finding":"The IGFBP4 gene core promoter activity resides downstream of position -289 and upstream of -6; cAMP response elements between positions -869 and -6 mediate a ~2-fold increase in promoter activity upon dibutyryl-cAMP stimulation, consistent with PTH- and cAMP-dependent regulation of IGFBP-4 expression in osteoblasts.","method":"Deletion mutagenesis of IGFBP4 promoter constructs, reporter assay, TATA box and transcription start site mapping","journal":"Genomics","confidence":"Medium","confidence_rationale":"Tier 1 — deletion mutagenesis of promoter with reporter assay, single study","pmids":["9615225"],"is_preprint":false},{"year":1997,"finding":"Surface-bound plasmin (generated by exogenous u-PA/plasminogen at HT29-D4 colon carcinoma cell surface) selectively cleaved IGFBP-4 (>95%) among IGFBP-2, -4, and -6 present, generating 18- and 14-kDa fragments with poor IGF-II affinity; approximately 20% of 125I-IGF-II released from IGFBP complexes was transferred to cell-surface IGF-I receptors.","method":"Cell surface plasmin activation assay, Western ligand blot, 125I-IGF-II transfer to IGF-I receptor measurement","journal":"International journal of cancer","confidence":"Medium","confidence_rationale":"Tier 2 — cell-based protease mechanism with receptor binding readout, moderate evidence from single study","pmids":["9311602"],"is_preprint":false},{"year":2011,"finding":"GATA-4 overexpression in mesenchymal stromal cells significantly upregulated IGFBP-4 expression; knockdown of IGFBP-4 reduced the cardiomyogenic transdifferentiation rate of GATA-4-overexpressing MSCs, demonstrating that IGFBP-4 acts downstream of GATA-4 to promote myocardial differentiation.","method":"Retroviral GATA-4 overexpression, IGFBP-4 siRNA knockdown, flow cytometry for α-sarcomeric actinin, electrophysiologic recording","journal":"Cytotherapy","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis by knockdown within overexpression system, functional cellular readout","pmids":["21846294"],"is_preprint":false},{"year":2023,"finding":"MEX3A, an RNA-binding protein, binds IGFBP4 mRNA in breast cancer cells and decreases IGFBP4 mRNA levels, thereby reducing IGFBP-4 protein and activating PI3K/AKT and downstream signaling pathways promoting cell cycle progression and migration.","method":"RNA pull-down, RNA immunoprecipitation (RIP), MEX3A knockdown/overexpression, xenograft tumor model, Western blot for PI3K/AKT pathway","journal":"Breast cancer research and treatment","confidence":"Medium","confidence_rationale":"Tier 2 — RNA pull-down plus RIP identifying direct mRNA binding, with functional consequence, single lab","pmids":["37433992"],"is_preprint":false},{"year":2022,"finding":"m6A methylation of IGFBP4 mRNA by METTL3 promotes YTHDF1-mediated IGFBP4 translation; reduced m6A methylation in endometrial cancer cells decreased IGFBP4 protein, and overexpression of IGFBP4 partially reversed cancer cell malignancy caused by reduced m6A methylation through suppression of NF-κB, ERK, and AKT pathways.","method":"MeRIP-qPCR, m6A-seq, polysome profiling, Western blot, METTL3 knockdown, IGFBP4 overexpression rescue, in vivo tumor formation","journal":"Cell biology and toxicology","confidence":"Medium","confidence_rationale":"Tier 2 — MeRIP and polysome profiling mechanistically link m6A to IGFBP4 translation, single lab with multiple methods","pmids":["35971034"],"is_preprint":false},{"year":2014,"finding":"Inhibition of tumor-associated αvβ3 integrin in melanoma cells enhanced IGFBP-4 expression through a p38 MAPK and matrix metalloproteinase-dependent mechanism, leading to reduced melanoma growth and angiogenesis in vivo.","method":"αvβ3 integrin inhibition in melanoma cells, IGFBP-4 expression measurement, p38 MAPK inhibitor studies, in vivo tumor angiogenesis model","journal":"Angiogenesis","confidence":"Low","confidence_rationale":"Tier 3 — indirect regulatory mechanism with limited mechanistic detail, single study","pmids":["25249331"],"is_preprint":false},{"year":2001,"finding":"Truncation of WT1 (disrupting its DNA-binding zinc finger domain) results in downregulation of IGFBP4 expression, identified by cDNA macroarray analysis, suggesting IGFBP4 as a transcriptional target of WT1.","method":"Gene targeting to generate WT1 truncation, cDNA macroarray gene expression analysis","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 — single macroarray finding without direct promoter binding confirmation","pmids":["11573961"],"is_preprint":false}],"current_model":"IGFBP-4 is a secreted inhibitor of IGF-I and IGF-II action that sequesters these growth factors and prevents their binding to the type I IGF receptor; its primary protease is PAPP-A, a metzincin metalloproteinase that cleaves IGFBP-4 at a single site (Met135-Lys136 in humans) in an IGF-II-dependent manner (by binding IGFBP-4, not PAPP-A) to release bioavailable IGF; IGFBP-4 also has IGF-independent functions, including direct inhibition of canonical Wnt signaling by physically binding the co-receptors LRP6 and Frizzled-8 to block Wnt3A engagement, inhibition of cathepsin B via its C-terminal thyroglobulin type-1 domain, and regulation of adipogenesis, cardiomyocyte differentiation, and skeletal homeostasis as demonstrated by knockout and transgenic mouse models."},"narrative":{"teleology":[{"year":1995,"claim":"Establishing the core mechanism of action: IGFBP-4 was shown to inhibit IGF signaling by directly competing with the type I IGF receptor for IGF-I and IGF-II binding, resolving the question of whether IGFBP-4 acts at the receptor level or through other means.","evidence":"Radioligand binding assays using 125I-IGF-I/II and proliferation assays with IGF analogs of reduced IGFBP affinity in bone cells","pmids":["7544787"],"confidence":"High","gaps":["Relative affinities for IGF-I vs. IGF-II not precisely quantified in this study","Whether IGFBP-4 has IGF-independent cellular effects was not addressed"]},{"year":1999,"claim":"Identifying the proteolytic switch: the specific PAPP-A cleavage site (Met135-Lys136) was mapped in human IGFBP-4, and deletion mutagenesis revealed that a distal region (residues 94-119) maintains the conformation necessary for substrate recognition, establishing that regulated proteolysis converts IGFBP-4 from an IGF inhibitor to a vehicle for IGF release.","evidence":"N-terminal sequencing, mass spectrometry of fragments, deletion mutagenesis, and protease-resistant mutants tested in osteoblast proliferation assays","pmids":["7538115","10620067"],"confidence":"High","gaps":["Three-dimensional structure of the IGFBP-4/IGF/PAPP-A ternary complex not determined","Contribution of glycosylation to protease susceptibility not fully resolved"]},{"year":2001,"claim":"Identifying PAPP-A as the physiological IGFBP-4 protease and demonstrating that IGF-II binding to IGFBP-4 (not to PAPP-A) is the conformational trigger for efficient proteolysis resolved the long-standing question of what enzyme controlled IGFBP-4 turnover and how IGF-dependence operates.","evidence":"Immunodepletion/immunoinhibition of PAPP-A from pregnancy serum, trophoblast and osteoblast conditioned media; pre-incubation experiments with IGF-binding-deficient IGFBP-4 mutants","pmids":["11522292","10898936","11158056","11994388","15541345"],"confidence":"High","gaps":["Whether other proteases (e.g., plasmin) contribute significantly to IGFBP-4 turnover in vivo remains unclear","Structural basis of IGF-II-induced conformational change in IGFBP-4 not resolved"]},{"year":1997,"claim":"A regulatory layer above PAPP-A was uncovered: basic C-terminal peptides of IGFBP-3, -5, and -6 inhibit IGFBP-4 proteolysis through heparin-reversible interactions, establishing that other IGFBPs modulate IGFBP-4 half-life and thus IGF bioavailability.","evidence":"Cell-free protease assay with 125I-IGFBP-4, synthetic peptide competition, and heparin reversal in osteoblast conditioned medium","pmids":["7499205","9165012"],"confidence":"High","gaps":["In vivo relevance of cross-IGFBP regulation of proteolysis not demonstrated","Whether this inhibition targets PAPP-A specifically or the substrate was not resolved"]},{"year":2001,"claim":"In vivo functional validation showed that the IGFBP-4/protease axis controls IGF bioavailability in bone: systemic wild-type IGFBP-4 increased bone formation and serum free IGF-I, but protease-resistant IGFBP-4 did not, proving that proteolytic release of IGF is the mechanism by which circulating IGFBP-4 stimulates osteogenesis.","evidence":"Systemic administration of wild-type vs. protease-resistant IGFBP-4 in mice with measurement of serum osteocalcin, ALP, and free IGF-I","pmids":["11356715"],"confidence":"High","gaps":["Tissue-specific contributions of endogenous PAPP-A to this effect not dissected","Whether other IGFBP-4-bound ligands contribute to the bone phenotype is unknown"]},{"year":2002,"claim":"Transgenic models demonstrated that IGFBP-4 is a physiological antagonist of IGF-I in smooth muscle: SMC-targeted overexpression caused smooth muscle hypoplasia that was epistatic to IGF-I overexpression, and protease-resistant IGFBP-4 transgenes produced more severe growth inhibition, confirming the protease-dependent release model in vivo.","evidence":"Transgenic and double-transgenic mice with SMC-specific IGFBP-4 and IGF-I expression; protease-resistant mutant transgene comparison","pmids":["9564877","11923290"],"confidence":"High","gaps":["Whether Wnt-inhibitory functions of IGFBP-4 contribute to the smooth muscle phenotype was not tested","Effects on smooth muscle function (contractility) beyond mass not assessed"]},{"year":2008,"claim":"A paradigm shift: IGFBP-4 was found to possess IGF-independent activity as a direct inhibitor of canonical Wnt signaling, physically binding LRP6 and Frizzled-8 to block Wnt3A engagement—this explained its cardiogenic activity and opened a new functional axis distinct from IGF sequestration.","evidence":"Co-immunoprecipitation of IGFBP-4 with LRP6 and Frz8, IGF-binding-deficient mutant retaining Wnt inhibition, Igfbp4 knockdown blocking cardiomyocyte differentiation in vitro and in Xenopus","pmids":["18528331"],"confidence":"High","gaps":["Binding interface between IGFBP-4 and LRP6/Frz8 not structurally resolved","Whether Wnt inhibition is relevant outside cardiac differentiation contexts not established"]},{"year":2013,"claim":"The C-terminal thyroglobulin type-1 domain was identified as an autonomous anti-angiogenic module that inhibits cathepsin B after cell internalization and reduces glioblastoma growth, revealing an IGF- and Wnt-independent effector function of IGFBP-4 proteolytic fragments.","evidence":"Recombinant C-terminal IGFBP-4 fragment tested in cathepsin B activity assay, EC tubulogenesis assay, and xenograft tumor model","pmids":["23633927"],"confidence":"High","gaps":["Whether PAPP-A-generated C-terminal fragments achieve sufficient local concentration for cathepsin B inhibition in vivo is unknown","Specificity for cathepsin B versus other cathepsins not fully tested"]},{"year":2016,"claim":"The Wnt-inhibitory mechanism was refined in the ischemic heart: IGFBP-4 targets β-catenin rather than LRP5/6 (unlike Dkk1), and direct intracardiac injection of recombinant IGFBP-4 was cardioprotective, establishing the molecular specificity and therapeutic relevance of IGFBP-4-mediated Wnt inhibition.","evidence":"Conditional cardiomyocyte-specific LRP5/6 and β-catenin knockout mice subjected to myocardial infarction, recombinant protein injection, Tcf reporter assay","pmids":["27803037"],"confidence":"High","gaps":["Reconciliation with the 2008 finding of LRP6/Frz8 binding is needed—different targets in different cell types possible but not tested","Long-term cardiac functional outcomes not reported"]},{"year":2017,"claim":"Global Igfbp4 knockout revealed in vivo requirements for adipogenesis and sex-specific skeletal homeostasis, establishing that IGFBP-4 is not simply redundant with other IGFBPs but has non-redundant roles in mesenchymal cell fate decisions including adipocyte differentiation (via Akt signaling) and osteoclastogenesis.","evidence":"Igfbp4 knockout mice analyzed by micro-CT, histomorphometry, primary BMSC/eMSC adipogenesis assays, high-fat diet and ovariectomy challenges","pmids":["28938423","28184001"],"confidence":"High","gaps":["Relative contributions of IGF-dependent vs. Wnt-dependent mechanisms to the knockout phenotypes not dissected","Compensatory changes in other IGFBPs in knockout mice not fully characterized"]},{"year":null,"claim":"Key unresolved questions include the structural basis of IGF-II-induced conformational change enabling PAPP-A cleavage, the relative in vivo contributions of IGF-sequestration versus Wnt-inhibition versus cathepsin B inhibition across tissues, and whether the apparently discrepant Wnt-pathway targets (LRP6/Frz8 vs. β-catenin) reflect context-dependent mechanisms.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of IGFBP-4 in complex with PAPP-A or Wnt pathway components","Tissue-specific dissection of IGF-dependent vs. IGF-independent functions lacking","No human genetic disease directly linked to IGFBP4 loss-of-function mutations"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,7,17]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[0,11]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,7]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,20,23]},{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[20,21]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[7,17]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[7,18,19]}],"complexes":[],"partners":["PAPP-A","LRP6","FZD8","IGF1","IGF2","CTNNB1"],"other_free_text":[]},"mechanistic_narrative":"IGFBP-4 is a secreted modulator of growth factor signaling that operates through both IGF-dependent and IGF-independent mechanisms to regulate cell proliferation, differentiation, and tissue homeostasis. Its canonical function is sequestration of IGF-I and IGF-II, preventing their binding to the type I IGF receptor and thereby inhibiting IGF-driven proliferation in bone, smooth muscle, and other tissues [PMID:7544787, PMID:9564877]. This inhibitory activity is regulated by proteolytic cleavage at Met135-Lys136 by the metalloproteinase PAPP-A, which requires prior IGF-II binding to IGFBP-4 to render it an efficient substrate, releasing bioavailable IGF [PMID:10620067, PMID:11522292, PMID:10898936]. Independent of IGF binding, IGFBP-4 inhibits canonical Wnt/β-catenin signaling by physically engaging co-receptors LRP6 and Frizzled-8 to block Wnt3A, a mechanism that promotes cardiomyocyte differentiation and protects against ischemic cardiac injury, and its C-terminal thyroglobulin type-1 domain inhibits cathepsin B, conferring anti-angiogenic activity [PMID:18528331, PMID:27803037, PMID:23633927]."},"prefetch_data":{"uniprot":{"accession":"P22692","full_name":"Insulin-like growth factor-binding protein 4","aliases":[],"length_aa":258,"mass_kda":27.9,"function":"IGF-binding proteins prolong the half-life of the IGFs and have been shown to either inhibit or stimulate the growth promoting effects of the IGFs on cell culture. 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thereby insulin-like growth factor bioavailability, in response to proinflammatory cytokine exposure in vitro.","date":"2013","source":"Connective tissue research","url":"https://pubmed.ncbi.nlm.nih.gov/24060054","citation_count":18,"is_preprint":false},{"pmid":"26025773","id":"PMC_26025773","title":"Characterization of endogenously circulating IGFBP-4 fragments-Novel biomarkers for cardiac risk assessment.","date":"2015","source":"Clinical biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/26025773","citation_count":17,"is_preprint":false},{"pmid":"35971034","id":"PMC_35971034","title":"m6A mRNA methylation regulates the ERK/NF-κB/AKT signaling pathway through the PAPPA/IGFBP4 axis to promote proliferation and tumor formation in endometrial cancer.","date":"2022","source":"Cell biology and toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/35971034","citation_count":17,"is_preprint":false},{"pmid":"34177367","id":"PMC_34177367","title":"IGFBP-4: A promising biomarker for lung cancer.","date":"2021","source":"Journal of medical biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/34177367","citation_count":17,"is_preprint":false},{"pmid":"9615225","id":"PMC_9615225","title":"Structure and transcription regulation of the human insulin-like growth factor binding protein 4 gene (IGFBP4).","date":"1998","source":"Genomics","url":"https://pubmed.ncbi.nlm.nih.gov/9615225","citation_count":17,"is_preprint":false},{"pmid":"31159802","id":"PMC_31159802","title":"The IGF system in patients with inflammatory bowel disease treated with prednisolone or infliximab: potential role of the stanniocalcin-2 / PAPP-A / IGFBP-4 axis.","date":"2019","source":"BMC gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/31159802","citation_count":16,"is_preprint":false},{"pmid":"11517150","id":"PMC_11517150","title":"Distribution of chimeric IGF binding protein (IGFBP)-3 and IGFBP-4 in the rat heart: importance of C-terminal basic region.","date":"2001","source":"Endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/11517150","citation_count":16,"is_preprint":false},{"pmid":"9080428","id":"PMC_9080428","title":"MK-801 inhibits the cortical increase in IGF-1, IGFBP-2 and IGFBP-4 expression following trauma.","date":"1997","source":"Neuroreport","url":"https://pubmed.ncbi.nlm.nih.gov/9080428","citation_count":16,"is_preprint":false},{"pmid":"23603559","id":"PMC_23603559","title":"Rescue of silenced UCHL1 and IGFBP4 expression suppresses clonogenicity of giant cell tumor-derived stromal cells.","date":"2013","source":"Cancer letters","url":"https://pubmed.ncbi.nlm.nih.gov/23603559","citation_count":15,"is_preprint":false},{"pmid":"34506805","id":"PMC_34506805","title":"Metabolic improvement after gastric bypass correlates with changes in IGF-regulatory proteins stanniocalcin-2 and IGFBP-4.","date":"2021","source":"Metabolism: clinical and experimental","url":"https://pubmed.ncbi.nlm.nih.gov/34506805","citation_count":14,"is_preprint":false},{"pmid":"28964325","id":"PMC_28964325","title":"Physiological parameters regulating circulating levels of the IGFBP-4/Stanniocalcin-2/PAPP-A axis.","date":"2017","source":"Metabolism: clinical and experimental","url":"https://pubmed.ncbi.nlm.nih.gov/28964325","citation_count":14,"is_preprint":false},{"pmid":"26243597","id":"PMC_26243597","title":"Serum IGFBP4 concentration decreased in dairy heifers towards day 18 of pregnancy.","date":"2015","source":"Journal of veterinary science","url":"https://pubmed.ncbi.nlm.nih.gov/26243597","citation_count":14,"is_preprint":false},{"pmid":"32760196","id":"PMC_32760196","title":"LncRNA IGFBP4-1 promotes tumor development by activating Janus kinase-signal transducer and activator of transcription pathway in bladder urothelial carcinoma.","date":"2020","source":"International journal of biological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/32760196","citation_count":14,"is_preprint":false},{"pmid":"11052994","id":"PMC_11052994","title":"Regulation of IGFBP-4 levels in human intestinal muscle by an IGF-I-activated, confluence-dependent protease.","date":"2000","source":"American journal of physiology. Gastrointestinal and liver physiology","url":"https://pubmed.ncbi.nlm.nih.gov/11052994","citation_count":13,"is_preprint":false},{"pmid":"31930684","id":"PMC_31930684","title":"Significant polyomic and functional upregulation of the PAPP-A/IGFBP-4/5/IGF-1 axis in chronic rhinosinusitis with nasal polyps.","date":"2020","source":"International forum of allergy & rhinology","url":"https://pubmed.ncbi.nlm.nih.gov/31930684","citation_count":12,"is_preprint":false},{"pmid":"26315127","id":"PMC_26315127","title":"Comparative RNA-seq analysis of the Tritrichomonas foetus PIG30/1 isolate from pigs reveals close association with Tritrichomonas foetus BP-4 isolate 'bovine genotype'.","date":"2015","source":"Veterinary parasitology","url":"https://pubmed.ncbi.nlm.nih.gov/26315127","citation_count":12,"is_preprint":false},{"pmid":"32597006","id":"PMC_32597006","title":"IGFBP-4 enhances VEGF-induced angiogenesis in a mouse model of myocardial infarction.","date":"2020","source":"Journal of cellular and molecular medicine","url":"https://pubmed.ncbi.nlm.nih.gov/32597006","citation_count":12,"is_preprint":false},{"pmid":"15098919","id":"PMC_15098919","title":"Expression of insulin-like growth factor I (IGF-I) gene and of genes for IGF-binding proteins 1, 2, 3, 4 (IGFBP-1-IGFBP-4) in non-neoplastic human thyroid cells and in certain human thyroid cancers. Effect of exogenous IGF-I on this expression.","date":"2004","source":"Endocrine research","url":"https://pubmed.ncbi.nlm.nih.gov/15098919","citation_count":12,"is_preprint":false},{"pmid":"37762544","id":"PMC_37762544","title":"Increased Levels of Circulating IGFBP4 and ANGPTL8 with a Prospective Role in Diabetic Nephropathy.","date":"2023","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/37762544","citation_count":11,"is_preprint":false},{"pmid":"11573961","id":"PMC_11573961","title":"Truncation of WT1 results in downregulation of cyclin G1 and IGFBP-4 expression.","date":"2001","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/11573961","citation_count":11,"is_preprint":false},{"pmid":"29946296","id":"PMC_29946296","title":"Increased Concentrations of Insulin-Like Growth Factor Binding Protein (IGFBP)-2, IGFBP-3, and IGFBP-4 Are Associated With Fetal Mortality in Pregnant Cows.","date":"2018","source":"Frontiers in endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/29946296","citation_count":11,"is_preprint":false},{"pmid":"33061500","id":"PMC_33061500","title":"Effect of Anthocyanins Supplementation on Serum IGFBP-4 Fragments and Glycemic Control in Patients with Fasting Hyperglycemia: A Randomized Controlled Trial.","date":"2020","source":"Diabetes, metabolic syndrome and obesity : targets and therapy","url":"https://pubmed.ncbi.nlm.nih.gov/33061500","citation_count":10,"is_preprint":false},{"pmid":"36087491","id":"PMC_36087491","title":"Cytotoxicity of BP-3 and BP-4: Blockage of extrusion pumps, oxidative damage and programmed cell death on Chlamydomonas reinhardtii.","date":"2022","source":"Aquatic toxicology (Amsterdam, Netherlands)","url":"https://pubmed.ncbi.nlm.nih.gov/36087491","citation_count":10,"is_preprint":false},{"pmid":"9359468","id":"PMC_9359468","title":"Transforming growth factor-alpha stimulates insulin-like growth factor binding protein-4 (IGFBP-4) expression and blocks follicle-stimulating hormone regulation of IGFBP-4 production in rat granulosa cells.","date":"1997","source":"Molecular and cellular endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/9359468","citation_count":10,"is_preprint":false},{"pmid":"37433992","id":"PMC_37433992","title":"The RNA binding protein MEX3A promotes tumor progression of breast cancer by post-transcriptional regulation of IGFBP4.","date":"2023","source":"Breast cancer research and treatment","url":"https://pubmed.ncbi.nlm.nih.gov/37433992","citation_count":9,"is_preprint":false},{"pmid":"15541345","id":"PMC_15541345","title":"IGFBP-4 degradation by pregnancy-associated plasma protein-A in MC3T3 osteoblasts.","date":"2004","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/15541345","citation_count":9,"is_preprint":false},{"pmid":"17882566","id":"PMC_17882566","title":"Insulin-like growth factor binding protein (IGFBP)-mediated hair cell survival on the mouse utricle exposed to neomycin: the roles of IGFBP-4 and IGFBP-5.","date":"2007","source":"Acta oto-laryngologica. Supplementum","url":"https://pubmed.ncbi.nlm.nih.gov/17882566","citation_count":9,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":64003,"output_tokens":8156,"usd":0.157174},"stage2":{"model":"claude-opus-4-6","input_tokens":11885,"output_tokens":3264,"usd":0.211537},"total_usd":0.368711,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"IGFBP-4 inhibits IGF action by preventing binding of IGF-I and IGF-II to the type I IGF receptor; coincubation with IGFBP-4 decreased binding of both 125I-IGF-I and 125I-IGF-II to bone cells and to purified type I IGF receptor in a dose-dependent manner, and IGFBP-4 had no effect on proliferation induced by IGF analogs with >100-fold reduced affinity for IGFBP-4.\",\n      \"method\": \"Radioligand binding assays, cell proliferation assays with IGF analogs of reduced IGFBP affinity\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro receptor binding assay with affinity-reduced IGF analogs as controls, multiple orthogonal methods\",\n      \"pmids\": [\"7544787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"The protease that cleaves IGFBP-4 cuts at the peptide bond between residues 120-121 (Lys-120/Met-121) within the non-homologous mid-region of IGFBP-4; site-directed mutagenesis of K120A rendered IGFBP-4 relatively resistant to cleavage, and the resulting 16-kDa N-terminal fragment lost the ability to inhibit IGF-1-stimulated thymidine uptake.\",\n      \"method\": \"Electrospray mass spectrometry, amino acid sequencing, site-directed mutagenesis (K120A), thymidine incorporation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with purified protease, mutagenesis, and functional validation in one study\",\n      \"pmids\": [\"7538115\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The human osteoblast IGF-II-dependent IGFBP-4 protease cleaves at Met135-Lys136 as the sole cleavage site; deletion of residues 94-119 (which contain no cleavage site) blocked proteolysis, suggesting these residues maintain the IGFBP-4 conformation required to expose the cleavage site; protease-resistant analogs were more potent inhibitors of IGF-II-induced proliferation in osteoblasts (which produce the protease) than wild-type IGFBP-4.\",\n      \"method\": \"N-terminal amino acid sequencing, mass spectrometry of proteolytic fragments, deletion mutagenesis, cell proliferation assay\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple mutagenesis constructs, mass spectrometry, and functional cell assay\",\n      \"pmids\": [\"10620067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The interaction between IGF-II and IGFBP-4 (not between IGF-II and PAPP-A) is essential for efficient IGFBP-4 proteolysis by PAPP-A; pre-incubation of IGFBP-4 with IGF-II followed by removal of free IGF-II allowed proteolysis, while pre-incubation of PAPP-A with IGF-II did not; IGFBP-4 mutants lacking IGF-binding activity but retaining the cleavage site were resistant to proteolysis regardless of IGF-II presence.\",\n      \"method\": \"Cell-free protease assay, PAPP-A antibody immunoprecipitation, IGF-binding domain deletion mutants, pre-incubation experiments\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with pre-incubation experiments and PAPP-A-specific antibody depletion\",\n      \"pmids\": [\"10898936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PAPP-A is identified as the protease responsible for IGF-II-dependent IGFBP-4 cleavage in human pregnancy serum; cleavage of IGFBP-4 by PAPP-A can occur without IGF in the absence of IGF but at a very slow rate; IGF-II enhances proteolysis by binding to IGFBP-4 (making IGFBP-4 the substrate), not by directly interacting with PAPP-A.\",\n      \"method\": \"Specific PAPP-A antibody immunoprecipitation/immunodepletion of protease activity, cell-free proteolysis assay, IGF binding studies with purified recombinant proteins\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with highly purified recombinant proteins and antibody depletion confirming identity of protease\",\n      \"pmids\": [\"11522292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PAPP-A accounts for the predominant IGFBP-4 proteolytic activity in human pregnancy serum; immunoprecipitation with PAPP-A antibody completely abolished both IGF-II-dependent and IGF-II-independent IGFBP-4 proteolytic activity; serum PAPP-A enhanced IGF bioactivity in vitro by degrading IGFBP-4, an effect blocked by PAPP-A neutralizing antibody.\",\n      \"method\": \"Immunoprecipitation/immunodepletion of PAPP-A from pregnancy serum, cell proliferation assay with wild-type vs. protease-resistant IGFBP-4, PAPP-A neutralizing antibody\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — antibody depletion with functional readout, replicated across multiple methods\",\n      \"pmids\": [\"11158056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PAPP-A is the IGFBP-4 protease in human trophoblasts and decidualized endometrial stromal cells; IGFBP-4 protease activity was confirmed by specific immunoinhibition and immunodepletion with PAPP-A antibodies; proteolysis was IGF-II-dependent, and its physiological inhibitor proMBP was identified in trophoblast conditioned medium.\",\n      \"method\": \"Immunoinhibition, immunodepletion with specific PAPP-A antibodies, conditioned medium protease assay, ELISA for PAPP-A and proMBP\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal antibody methods (inhibition + depletion) identifying enzyme and inhibitor\",\n      \"pmids\": [\"11994388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"IGFBP-4 functions as an inhibitor of canonical Wnt signaling independent of its IGF-binding activity; IGFBP-4 physically interacts with Wnt co-receptor LRP6 and Frizzled-8 (Frz8), inhibiting Wnt3A binding to these receptors; this cardiogenic effect was attenuated by IGFs through IGFBP-4 sequestration.\",\n      \"method\": \"Co-immunoprecipitation of IGFBP-4 with LRP6 and Frz8, in vitro cardiomyocyte differentiation, Igfbp4 knockdown in vitro and in vivo, IGF-binding-deficient IGFBP-4 mutant, Wnt3A binding competition assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, loss-of-function, mutant protein, and in vivo validation in a high-impact journal\",\n      \"pmids\": [\"18528331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"IGFBP-3 functions as an IGF-reversible inhibitor of IGFBP-4 proteolysis; recombinant IGFBP-3 added to osteoblast conditioned medium inhibited 125I-IGFBP-4 proteolysis by 90%, and this inhibition was reversed by IGF-I or IGF-II; synthetic peptides spanning the highly basic, putative heparin-binding C-terminal region of IGFBP-3 mediated this inhibition, and heparin reversed the inhibitory effect.\",\n      \"method\": \"Cell-free protease assay with 125I-IGFBP-4, synthetic peptide competition, heparin reversal experiment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted cell-free assay with synthetic peptides and defined inhibitor mechanism\",\n      \"pmids\": [\"7499205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"IGFBP-3, -5, and -6 all inhibit IGFBP-4 proteolysis through homologous highly basic, heparin-binding domains within the conserved thyroglobulin type-1 motif at their C-termini; synthetic peptides from these regions inhibited 125I-IGFBP-4 proteolysis by MC3T3-E1 osteoblast conditioned medium, and heparin reversed their inhibitory effects.\",\n      \"method\": \"Cell-free protease assay with 125I-IGFBP-4, synthetic peptide competition assay, heparin binding studies, IC50 determination\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted cell-free assay with purified synthetic peptides, mechanistically defined\",\n      \"pmids\": [\"9165012\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Systemic administration of wild-type IGFBP-4 increased bone formation parameters in mice by increasing IGF bioavailability through proteolysis of IGFBP-4; protease-resistant IGFBP-4 administered systemically did not increase bone formation or serum free IGF-I, whereas wild-type IGFBP-4 did, confirming that the stimulatory effect requires IGFBP-4 protease-mediated release of IGF.\",\n      \"method\": \"In vivo systemic administration of wild-type vs. protease-resistant IGFBP-4, biochemical bone formation markers (serum osteocalcin, ALP), serum free IGF-I measurement, local administration assay\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo loss-of-function with protease-resistant mutant and multiple bone formation readouts\",\n      \"pmids\": [\"11356715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Overexpression of IGFBP-4 in smooth muscle cells (SMC) of transgenic mice caused smooth muscle hypoplasia in multiple SMC-rich organs, a reciprocal phenotype to IGF-I-overexpressing transgenic mice; IGFBP-4 overexpression was confirmed to be the functional antagonist of IGF-I in SMC in vivo.\",\n      \"method\": \"Transgenic mouse generation with SMC-specific promoter (SMP8-BP-4), tissue weight measurements, in situ hybridization, Western ligand blotting, double transgenic cross with SMP8-IGF-I mice\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in double transgenic model with phenotypic quantification\",\n      \"pmids\": [\"9564877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Protease-resistant IGFBP-4 (with substitutions 119-AAMAAVADASAMA-133 in the cleavage domain) is stabilized in vivo and causes greater growth inhibition of SMC-rich tissues than equivalent levels of native IGFBP-4, demonstrating that IGFBP-4 proteolysis is required for local IGF-I action in smooth muscle.\",\n      \"method\": \"Site-directed mutagenesis of IGFBP-4 cleavage domain, transgenic mice with SMC-specific expression, tissue weight measurements, Western blot quantification of IGFBP-4/transgene mRNA ratio\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — protease-resistant mutant validated in vivo, direct comparison with native IGFBP-4 transgenic line\",\n      \"pmids\": [\"11923290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"PAPP-A is the IGFBP-4 protease in MC3T3-E1 osteoblasts; immunodepletion of PAPP-A from MC3T3-E1 conditioned medium abolished IGFBP-4 degradation; IGFBP-4 degradation was enhanced by IGF-II and inhibited by mutation of basic residues at/near the PAPP-A cleavage site.\",\n      \"method\": \"Immunodepletion of PAPP-A from conditioned medium, IGFBP-4 degradation assay, site-directed mutagenesis of IGFBP-4 cleavage site, RT-PCR for PAPP-A mRNA\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — immunodepletion plus mutagenesis confirming PAPP-A as the osteoblast IGFBP-4 protease\",\n      \"pmids\": [\"15541345\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Paracrine overexpression of IGFBP-4 in osteoblasts via an osteocalcin promoter transgene caused 25-fold increased calvaria IGFBP-4, reduced bone turnover (osteoblast number and bone formation rate ~50% of controls), global growth retardation, and disproportionate reduction of skeletal weight, attributed to IGF-1 sequestration.\",\n      \"method\": \"Transgenic mice (OC-BP4), Western ligand blot, quantitative bone histomorphometry, organ allometry\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-targeted transgenic model with quantitative histomorphometry and phenotypic characterization\",\n      \"pmids\": [\"12733722\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SOX9 directly binds the IGFBP-4 promoter and transcriptionally activates IGFBP-4 expression in intestinal epithelial cells; IGFBP-4 was significantly downregulated in Sox9-deficient crypts and adenoma cells; SOX9-induced inhibition of cell proliferation was restored by a neutralizing IGFBP-4 antibody.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP) of SOX9 at IGFBP-4 promoter, reporter assay, Sox9 knockout mouse model, neutralizing antibody rescue experiment, immunolocalization\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP plus reporter assay plus loss-of-function rescue defines direct transcriptional regulation\",\n      \"pmids\": [\"23660500\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The anti-angiogenic and anti-tumorigenic activities of IGFBP-4 are located in its C-terminal fragment (CIBP-4) containing a thyroglobulin type-1 (Tg1) domain; CIBP-4 internalized into endothelial cells and glioblastoma cells, co-localizing with lysosomal structures, inhibited cathepsin B (CatB) activity, blocked EC tubulogenesis, and reduced glioblastoma tumor growth by 60% in vivo.\",\n      \"method\": \"C-terminal IGFBP-4 fragment production, in vitro cathepsin B activity assay, Matrigel EC tubulogenesis assay, in vivo biodistribution with Cy5.5 labeling, xenograft tumor model\",\n      \"journal\": \"Neoplasia (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vitro enzyme assay plus in vivo tumor model with mechanistic domain mapping\",\n      \"pmids\": [\"23633927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"IGFBP-4 inhibits β-catenin signaling in the ischemic heart by targeting β-catenin (not LRP5/6), whereas Dkk1 exerts injury mainly by inducing LRP5/6 endocytosis and degradation; direct injection of recombinant IGFBP-4 into infarcted hearts protected against ischemic injury through β-catenin inhibition.\",\n      \"method\": \"Conditional cardiomyocyte-specific LRP5/6 and β-catenin knockout mice, surgical myocardial infarction model, recombinant protein injection, Tcf reporter assay\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with conditional knockouts plus recombinant protein intervention in vivo\",\n      \"pmids\": [\"27803037\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IGFBP-4 is required for adipogenesis; Igfbp4-null mice had decreased inguinal and gonadal white adipose tissue weight, reduced Pparγ expression, and impaired adipogenic differentiation of primary bone marrow stromal cells (BMSCs) and ear mesenchymal stem cells (eMSCs); loss of Igfbp4 blunted phosphorylated Akt increases during adipogenesis.\",\n      \"method\": \"Igfbp4 knockout mice, primary BMSC and eMSC culture adipogenesis assay, high-fat diet challenge, ovariectomy, Western blot for p-Akt\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic knockout with defined cellular and signaling phenotype across multiple assays\",\n      \"pmids\": [\"28938423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Igfbp4-null mice exhibit marked sex-specific skeletal phenotypes; female knockouts had reduced aBMD/aBMC, lower trabecular BV/TV, higher osteoclastogenesis, and lower bone formation, while males had increased trabeculae with higher connectivity density and reduced mineralized surface; loss of Igfbp4 modulates mesenchymal stromal cell differentiation and osteoclastogenesis in a gender-specific manner.\",\n      \"method\": \"Igfbp4 knockout mice, micro-CT, histomorphometry, osteoblast/osteoclast culture assays, sclerostin ELISA\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — comprehensive knockout analysis with histomorphometry and cell culture mechanistic follow-up\",\n      \"pmids\": [\"28184001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"IGFBP-4 purified from bovine pulmonary artery endothelial cells crossed the capillary boundary of the isolated perfused rat heart and distributed preferentially in subendothelial connective tissue (connective tissue/muscle ratio ~20-27:1), unlike IGFBP-1, -2, -3 and IGF-I which preferentially localized in cardiac muscle.\",\n      \"method\": \"Perfused isolated beating rat heart, purified glycosylated and nonglycosylated IGFBP-4, tissue distribution quantification\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment in perfused organ model, single study\",\n      \"pmids\": [\"1377125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The C-terminal basic region of IGFBP-4 determines its connective tissue distribution in the rat heart; chimeric IGFBP-4(3) (IGFBP-4 carrying the IGFBP-3 C-terminal basic region) redistributed to cardiac muscle like IGFBP-3, and IGFBP-3(4) localized in connective tissue like IGFBP-4.\",\n      \"method\": \"Chimeric IGFBP-3/4 protein construction, perfused rat heart distribution assay, microvascular endothelial cell binding assay\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain-swap chimeras with functional tissue distribution assay, single study\",\n      \"pmids\": [\"11517150\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"IGFBP-4 in the IGFBP-4 protease system of human osteoblast-like cells is regulated by TGF-β, which augmented IGF-II-dependent IGFBP-4 proteolysis and decreased IGFBP-4 mRNA expression; other factors (PTH, GH, insulin, calcitonin, glucocorticoids, sex steroids, 1,25-dihydroxyvitamin D3, EGF) had no significant effect on IGFBP-4 protease activity.\",\n      \"method\": \"Cell-free IGFBP-4 protease activity assay, Western ligand blotting with densitometry, Northern analysis for mRNA\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-free protease assay with systematic hormone testing, moderate evidence from single lab\",\n      \"pmids\": [\"7527411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Circulating N-terminal (Asp1-Phe122) and C-terminal (Lys136-Glu237) fragments of IGFBP-4 generated by in vivo proteolysis were isolated from human hemofiltrate; the N-terminal fragment retained significant IGF binding (Kd IGF-II=17 nM, IGF-I=5 nM), while the C-terminal fragment had very low IGF affinity (Kd IGF-II=690 nM, IGF-I>60 nM); disulfide bonding pattern of C-terminal fragment: 153-183, 194-205, 207-228.\",\n      \"method\": \"Affinity purification from human hemofiltrate, mass spectrometry, Edman sequencing, plasmon resonance spectroscopy, ligand blot, saturation and displacement binding studies, proteolytic digestion for disulfide mapping\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical characterization with multiple orthogonal methods, structural data from natural human samples\",\n      \"pmids\": [\"10819974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The IGFBP4 gene core promoter activity resides downstream of position -289 and upstream of -6; cAMP response elements between positions -869 and -6 mediate a ~2-fold increase in promoter activity upon dibutyryl-cAMP stimulation, consistent with PTH- and cAMP-dependent regulation of IGFBP-4 expression in osteoblasts.\",\n      \"method\": \"Deletion mutagenesis of IGFBP4 promoter constructs, reporter assay, TATA box and transcription start site mapping\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — deletion mutagenesis of promoter with reporter assay, single study\",\n      \"pmids\": [\"9615225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Surface-bound plasmin (generated by exogenous u-PA/plasminogen at HT29-D4 colon carcinoma cell surface) selectively cleaved IGFBP-4 (>95%) among IGFBP-2, -4, and -6 present, generating 18- and 14-kDa fragments with poor IGF-II affinity; approximately 20% of 125I-IGF-II released from IGFBP complexes was transferred to cell-surface IGF-I receptors.\",\n      \"method\": \"Cell surface plasmin activation assay, Western ligand blot, 125I-IGF-II transfer to IGF-I receptor measurement\",\n      \"journal\": \"International journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-based protease mechanism with receptor binding readout, moderate evidence from single study\",\n      \"pmids\": [\"9311602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"GATA-4 overexpression in mesenchymal stromal cells significantly upregulated IGFBP-4 expression; knockdown of IGFBP-4 reduced the cardiomyogenic transdifferentiation rate of GATA-4-overexpressing MSCs, demonstrating that IGFBP-4 acts downstream of GATA-4 to promote myocardial differentiation.\",\n      \"method\": \"Retroviral GATA-4 overexpression, IGFBP-4 siRNA knockdown, flow cytometry for α-sarcomeric actinin, electrophysiologic recording\",\n      \"journal\": \"Cytotherapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis by knockdown within overexpression system, functional cellular readout\",\n      \"pmids\": [\"21846294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"MEX3A, an RNA-binding protein, binds IGFBP4 mRNA in breast cancer cells and decreases IGFBP4 mRNA levels, thereby reducing IGFBP-4 protein and activating PI3K/AKT and downstream signaling pathways promoting cell cycle progression and migration.\",\n      \"method\": \"RNA pull-down, RNA immunoprecipitation (RIP), MEX3A knockdown/overexpression, xenograft tumor model, Western blot for PI3K/AKT pathway\",\n      \"journal\": \"Breast cancer research and treatment\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — RNA pull-down plus RIP identifying direct mRNA binding, with functional consequence, single lab\",\n      \"pmids\": [\"37433992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"m6A methylation of IGFBP4 mRNA by METTL3 promotes YTHDF1-mediated IGFBP4 translation; reduced m6A methylation in endometrial cancer cells decreased IGFBP4 protein, and overexpression of IGFBP4 partially reversed cancer cell malignancy caused by reduced m6A methylation through suppression of NF-κB, ERK, and AKT pathways.\",\n      \"method\": \"MeRIP-qPCR, m6A-seq, polysome profiling, Western blot, METTL3 knockdown, IGFBP4 overexpression rescue, in vivo tumor formation\",\n      \"journal\": \"Cell biology and toxicology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — MeRIP and polysome profiling mechanistically link m6A to IGFBP4 translation, single lab with multiple methods\",\n      \"pmids\": [\"35971034\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Inhibition of tumor-associated αvβ3 integrin in melanoma cells enhanced IGFBP-4 expression through a p38 MAPK and matrix metalloproteinase-dependent mechanism, leading to reduced melanoma growth and angiogenesis in vivo.\",\n      \"method\": \"αvβ3 integrin inhibition in melanoma cells, IGFBP-4 expression measurement, p38 MAPK inhibitor studies, in vivo tumor angiogenesis model\",\n      \"journal\": \"Angiogenesis\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — indirect regulatory mechanism with limited mechanistic detail, single study\",\n      \"pmids\": [\"25249331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Truncation of WT1 (disrupting its DNA-binding zinc finger domain) results in downregulation of IGFBP4 expression, identified by cDNA macroarray analysis, suggesting IGFBP4 as a transcriptional target of WT1.\",\n      \"method\": \"Gene targeting to generate WT1 truncation, cDNA macroarray gene expression analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single macroarray finding without direct promoter binding confirmation\",\n      \"pmids\": [\"11573961\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IGFBP-4 is a secreted inhibitor of IGF-I and IGF-II action that sequesters these growth factors and prevents their binding to the type I IGF receptor; its primary protease is PAPP-A, a metzincin metalloproteinase that cleaves IGFBP-4 at a single site (Met135-Lys136 in humans) in an IGF-II-dependent manner (by binding IGFBP-4, not PAPP-A) to release bioavailable IGF; IGFBP-4 also has IGF-independent functions, including direct inhibition of canonical Wnt signaling by physically binding the co-receptors LRP6 and Frizzled-8 to block Wnt3A engagement, inhibition of cathepsin B via its C-terminal thyroglobulin type-1 domain, and regulation of adipogenesis, cardiomyocyte differentiation, and skeletal homeostasis as demonstrated by knockout and transgenic mouse models.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"IGFBP-4 is a secreted modulator of growth factor signaling that operates through both IGF-dependent and IGF-independent mechanisms to regulate cell proliferation, differentiation, and tissue homeostasis. Its canonical function is sequestration of IGF-I and IGF-II, preventing their binding to the type I IGF receptor and thereby inhibiting IGF-driven proliferation in bone, smooth muscle, and other tissues [PMID:7544787, PMID:9564877]. This inhibitory activity is regulated by proteolytic cleavage at Met135-Lys136 by the metalloproteinase PAPP-A, which requires prior IGF-II binding to IGFBP-4 to render it an efficient substrate, releasing bioavailable IGF [PMID:10620067, PMID:11522292, PMID:10898936]. Independent of IGF binding, IGFBP-4 inhibits canonical Wnt/β-catenin signaling by physically engaging co-receptors LRP6 and Frizzled-8 to block Wnt3A, a mechanism that promotes cardiomyocyte differentiation and protects against ischemic cardiac injury, and its C-terminal thyroglobulin type-1 domain inhibits cathepsin B, conferring anti-angiogenic activity [PMID:18528331, PMID:27803037, PMID:23633927].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Establishing the core mechanism of action: IGFBP-4 was shown to inhibit IGF signaling by directly competing with the type I IGF receptor for IGF-I and IGF-II binding, resolving the question of whether IGFBP-4 acts at the receptor level or through other means.\",\n      \"evidence\": \"Radioligand binding assays using 125I-IGF-I/II and proliferation assays with IGF analogs of reduced IGFBP affinity in bone cells\",\n      \"pmids\": [\"7544787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative affinities for IGF-I vs. IGF-II not precisely quantified in this study\", \"Whether IGFBP-4 has IGF-independent cellular effects was not addressed\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identifying the proteolytic switch: the specific PAPP-A cleavage site (Met135-Lys136) was mapped in human IGFBP-4, and deletion mutagenesis revealed that a distal region (residues 94-119) maintains the conformation necessary for substrate recognition, establishing that regulated proteolysis converts IGFBP-4 from an IGF inhibitor to a vehicle for IGF release.\",\n      \"evidence\": \"N-terminal sequencing, mass spectrometry of fragments, deletion mutagenesis, and protease-resistant mutants tested in osteoblast proliferation assays\",\n      \"pmids\": [\"7538115\", \"10620067\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Three-dimensional structure of the IGFBP-4/IGF/PAPP-A ternary complex not determined\", \"Contribution of glycosylation to protease susceptibility not fully resolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identifying PAPP-A as the physiological IGFBP-4 protease and demonstrating that IGF-II binding to IGFBP-4 (not to PAPP-A) is the conformational trigger for efficient proteolysis resolved the long-standing question of what enzyme controlled IGFBP-4 turnover and how IGF-dependence operates.\",\n      \"evidence\": \"Immunodepletion/immunoinhibition of PAPP-A from pregnancy serum, trophoblast and osteoblast conditioned media; pre-incubation experiments with IGF-binding-deficient IGFBP-4 mutants\",\n      \"pmids\": [\"11522292\", \"10898936\", \"11158056\", \"11994388\", \"15541345\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other proteases (e.g., plasmin) contribute significantly to IGFBP-4 turnover in vivo remains unclear\", \"Structural basis of IGF-II-induced conformational change in IGFBP-4 not resolved\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"A regulatory layer above PAPP-A was uncovered: basic C-terminal peptides of IGFBP-3, -5, and -6 inhibit IGFBP-4 proteolysis through heparin-reversible interactions, establishing that other IGFBPs modulate IGFBP-4 half-life and thus IGF bioavailability.\",\n      \"evidence\": \"Cell-free protease assay with 125I-IGFBP-4, synthetic peptide competition, and heparin reversal in osteoblast conditioned medium\",\n      \"pmids\": [\"7499205\", \"9165012\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of cross-IGFBP regulation of proteolysis not demonstrated\", \"Whether this inhibition targets PAPP-A specifically or the substrate was not resolved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"In vivo functional validation showed that the IGFBP-4/protease axis controls IGF bioavailability in bone: systemic wild-type IGFBP-4 increased bone formation and serum free IGF-I, but protease-resistant IGFBP-4 did not, proving that proteolytic release of IGF is the mechanism by which circulating IGFBP-4 stimulates osteogenesis.\",\n      \"evidence\": \"Systemic administration of wild-type vs. protease-resistant IGFBP-4 in mice with measurement of serum osteocalcin, ALP, and free IGF-I\",\n      \"pmids\": [\"11356715\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific contributions of endogenous PAPP-A to this effect not dissected\", \"Whether other IGFBP-4-bound ligands contribute to the bone phenotype is unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Transgenic models demonstrated that IGFBP-4 is a physiological antagonist of IGF-I in smooth muscle: SMC-targeted overexpression caused smooth muscle hypoplasia that was epistatic to IGF-I overexpression, and protease-resistant IGFBP-4 transgenes produced more severe growth inhibition, confirming the protease-dependent release model in vivo.\",\n      \"evidence\": \"Transgenic and double-transgenic mice with SMC-specific IGFBP-4 and IGF-I expression; protease-resistant mutant transgene comparison\",\n      \"pmids\": [\"9564877\", \"11923290\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Wnt-inhibitory functions of IGFBP-4 contribute to the smooth muscle phenotype was not tested\", \"Effects on smooth muscle function (contractility) beyond mass not assessed\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"A paradigm shift: IGFBP-4 was found to possess IGF-independent activity as a direct inhibitor of canonical Wnt signaling, physically binding LRP6 and Frizzled-8 to block Wnt3A engagement—this explained its cardiogenic activity and opened a new functional axis distinct from IGF sequestration.\",\n      \"evidence\": \"Co-immunoprecipitation of IGFBP-4 with LRP6 and Frz8, IGF-binding-deficient mutant retaining Wnt inhibition, Igfbp4 knockdown blocking cardiomyocyte differentiation in vitro and in Xenopus\",\n      \"pmids\": [\"18528331\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Binding interface between IGFBP-4 and LRP6/Frz8 not structurally resolved\", \"Whether Wnt inhibition is relevant outside cardiac differentiation contexts not established\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"The C-terminal thyroglobulin type-1 domain was identified as an autonomous anti-angiogenic module that inhibits cathepsin B after cell internalization and reduces glioblastoma growth, revealing an IGF- and Wnt-independent effector function of IGFBP-4 proteolytic fragments.\",\n      \"evidence\": \"Recombinant C-terminal IGFBP-4 fragment tested in cathepsin B activity assay, EC tubulogenesis assay, and xenograft tumor model\",\n      \"pmids\": [\"23633927\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PAPP-A-generated C-terminal fragments achieve sufficient local concentration for cathepsin B inhibition in vivo is unknown\", \"Specificity for cathepsin B versus other cathepsins not fully tested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"The Wnt-inhibitory mechanism was refined in the ischemic heart: IGFBP-4 targets β-catenin rather than LRP5/6 (unlike Dkk1), and direct intracardiac injection of recombinant IGFBP-4 was cardioprotective, establishing the molecular specificity and therapeutic relevance of IGFBP-4-mediated Wnt inhibition.\",\n      \"evidence\": \"Conditional cardiomyocyte-specific LRP5/6 and β-catenin knockout mice subjected to myocardial infarction, recombinant protein injection, Tcf reporter assay\",\n      \"pmids\": [\"27803037\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Reconciliation with the 2008 finding of LRP6/Frz8 binding is needed—different targets in different cell types possible but not tested\", \"Long-term cardiac functional outcomes not reported\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Global Igfbp4 knockout revealed in vivo requirements for adipogenesis and sex-specific skeletal homeostasis, establishing that IGFBP-4 is not simply redundant with other IGFBPs but has non-redundant roles in mesenchymal cell fate decisions including adipocyte differentiation (via Akt signaling) and osteoclastogenesis.\",\n      \"evidence\": \"Igfbp4 knockout mice analyzed by micro-CT, histomorphometry, primary BMSC/eMSC adipogenesis assays, high-fat diet and ovariectomy challenges\",\n      \"pmids\": [\"28938423\", \"28184001\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of IGF-dependent vs. Wnt-dependent mechanisms to the knockout phenotypes not dissected\", \"Compensatory changes in other IGFBPs in knockout mice not fully characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of IGF-II-induced conformational change enabling PAPP-A cleavage, the relative in vivo contributions of IGF-sequestration versus Wnt-inhibition versus cathepsin B inhibition across tissues, and whether the apparently discrepant Wnt-pathway targets (LRP6/Frz8 vs. β-catenin) reflect context-dependent mechanisms.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of IGFBP-4 in complex with PAPP-A or Wnt pathway components\", \"Tissue-specific dissection of IGF-dependent vs. IGF-independent functions lacking\", \"No human genetic disease directly linked to IGFBP4 loss-of-function mutations\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 7, 17]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 11]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 20, 23]},\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [20, 21]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0162582\", \"supporting_discovery_ids\": [7, 17]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [7, 17]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [7, 18, 19]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"PAPP-A\", \"LRP6\", \"FZD8\", \"IGF1\", \"IGF2\", \"CTNNB1\"],\n    \"other_free_text\": []\n  }\n}\n```"}